(1) An example of a conventional liquid crystal display system is shown in FIG. 37. Such liquid crystal display system is generally provided with a dot-like printed layer, a white reflecting sheet or the like for irregular reflection at the back side of a light guide 1400, which guides the light from a light source 1401 to a liquid crystal display panel 1402, to obtain an uniform light intensity. In this case, since the light source emits light nondirectionally, the light can be uniformly diffused.
However, since the light is diffused, light directivity is lowered to lead to a low light intensity.
When the light intensity is increased, the light will be used inefficiently because the light is widely diffused, causing large power consumption. The lighting apparatus having such problems is not fit for long-time usage and not appropriate to portable electronic equipment.
When it is tried to lower the power consumption, however, the light intensity is also lowered.
(2) In order to solve these problems, it has been tried to increase directivity of the light from the light guide. Some known examples can be found in Japanese Patent Application Laid-open Nos. 5-089827, 6-067004, 6-130387,or the like. A prism array having fine prisms is used in these references to increase the light directivity. Each of the prisms has an apex which points the liquid crystal panel, and an apex angle is substantially 90.degree.. By supposing the light source to be the point light source, considering the optical path, and controlling the apex angle, the light directivity can be increased.
When a prism array is used, the light directivity can be increased. However, since part of the light is reflected at the prism array due to the total internal reflection and is returned to the light guide to lower the light intensity.
Moreover, when the liquid crystal display panel is viewed at an oblique angle, the display looks dark. In other words, a wide viewing angle range cannot be obtained. Such display system cannot be applied to a color television, for example, because a display of television requires a uniform and high light intensity and a wide viewing angle range.
(3) Another example of the liquid crystal display system which uses a prism array for increasing the light directivity is disclosed in Japanese Patent Application Laid-Open No. 2-17. Part of this system is shown in FIG. 38.
In this reference, the prism array efficiently changes the direction of most of the input light to a direction substantially perpendicular to the liquid crystal display panel, when the apex angle of each prism is 60.degree. and the normal to the liquid crystal display panel is in a position to halve the apex angle. In other words, each of prisms 1420p of a prism array 1420 has a vertical section which is an isosceles triangle, and its apex angle is 60.degree. (the angle .theta.=30.degree.), as shown in the figure. When light 1421r is output from the light guide at the angle .beta.=60.degree., an available maximum light intensity can be obtained. When the light 1421r is output at the angle .alpha.=70.degree., which is 10.degree. more than the angle .beta., or at the angle .gamma.=50.degree., which is 10.degree. less than the angle .beta., the light intensity is nearly half an available maximum value.
However, the light beam c which has been output at the angle .gamma. is then reflected at an output surface 1420e of the prism array 1420 toward the light guide 1421 by the total internal reflection. Consequently, the transmittance of the light with respect to the prism array is low, and the light cannot be utilized efficiently.
Moreover, there is another example of the prism array in the above reference. In order to output the light from the prism array at an angle of 15.degree. with respect to the normal to the liquid crystal display panel, the apex angle of each prism is set to 57.degree.. Each prism comprises two surfaces which form the apex, and one surface is inclined at an angle of 35.degree., and another surface is inclined at an angle of 22.degree. with respect to the normal to the liquid crystal display panel.
Even if the apex angle is controlled as described, however, there is the light which is reflected like the light beam c having the output angle .gamma., so that the light transmittance is not improved. In above-mentioned Japanese Patent Application Laid-Open No. 6-067004, the apex angle of the prism is controlled in the range of 70.degree. to 110.degree.. Also in this case, the above-mentioned problems still remain.
In the above-mentioned Japanese Patent Application Laid-Open No. 2-17, the shape of lenses provided on the light guide is modified. However, since the light is output at an angle of between about 60.degree. and about 80.degree. with respect to the light-output-side surface of the light guide, the problems as described are not solved independent of the lens shape, and the light directivity is low.
The lenses of the above reference turn the light to a particular direction to obtain improved directivity and intensity. In this case, the light has to be output from the light guide at an angle of 60.degree. with respect to the normal to the liquid crystal display panel. Therefore, the above-mentioned problems still remain.
Note that the light-output-side surface of the light guide is not flat because a plurality of lenses are arranged on the light guide. The uneven surface of the light guide lowers the utilization efficiency of the light.
(4) Other examples of the liquid crystal display system are disclosed in Japanese Patent Application Laid-Open Nos. 57-054926 and 58-095780, for example. Respective liquid crystal display system are shown in FIGS. 39A and 39B.
In the example of FIG. 39A, a reflecting plate 1433 is placed at the back of a liquid crystal display panel 1432 to utilize light 1436 from outside. Because this system utilizes only reflected light, a sufficient contrast cannot be obtained, so that it cannot be used in a dark place.
In the example of FIG. 39B, a transflector (semitransparent plate) 1434 and a lighting apparatus 1430 are provided at the back of the liquid crystal display panel 1432. When the display system is in a bright place, the light from outside is used, and when the display system is in a dark place, the light from the lighting apparatus is used. Neither the outside light 1436 nor backlight 1435 can give a sufficient brightness to the display since a transflector is used.
(5) There is also a lighting apparatus disclosed in PROCEEDINGS OF THE 15TH INTERNATIONAL DISPLAY RESEARCH CONFERENCE, pp. 735-738, as an example of the apparatus for improving utilization efficiency of the light from the light source.
The schematic sectional view of the apparatus is shown in FIG. 40. The light from a light source 1440 passes through a light guide 1441 to be diffused by a diffusing plate 1442. The diffused light is input to a polarizer 1443 which comprises a cholesteric liquid crystal layer 1444 and a quarter wave film 1445. Circularly-polarized light is reflected by the cholesteric liquid crystal layer when the direction of rotation of the circularly-polarized light is the same as that of helical structure of the cholesteric liquid crystal layer. When the direction of rotation of the circularly-polarized light is reverse to that of helical structure of the cholesteric liquid crystal layer, the circularly-polarized light can pass through it. Circularly-polarized light d which has passed the cholesteric liquid crystal layer is converted into linearly-polarized light by the quarter wave film 1445. Circularly-polarized light e which has been reflected at the cholesteric liquid crystal layer is returned to the diffusing plate 1442 to be diffused. Part of the diffused light rotates in the reverse direction of the helical structure of the cholesteric liquid crystal layer 1444, so that the light is again input to and now passes through it as light f.
However, part of the light e which has been reflected by the cholesteric liquid crystal layer 1444 is returned to the light guide 1441 through the diffusing plate 1442 to be absorbed by a diffuse reflecting film 1449 provided at the back of the light guide 1441 as light g. In this way, part of the light is wastefully lost to lower the light intensity and utilization efficiency.
Accordingly, it is an object of the invention to provide a lighting apparatus which can utilize the light efficiently to improve the light intensity with a wide viewing angle range, a liquid crystal display using such a lighting apparatus, and electronic equipment also using such a lighting apparatus.
Another object of the invention is to provide a lighting apparatus which can operate sufficiently with a small power consumption, a liquid crystal display using such a lighting apparatus, and electronic equipment also using such a lighting apparatus.
Still another object of the invention is to provide a lighting apparatus which can change the direction of the light from the light guide efficiently to a direction substantially parallel to the normal to the light-output-side surface of the light guide to improve the utilization efficiency, a liquid crystal display using such a lighting apparatus, and electronic equipment also using such a lighting apparatus.
Further object of the invention is to provide a lighting apparatus which can operate without lowering utilization efficiency of the light and reflecting function independent of the outside brightness, a liquid crystal display using such a lighting apparatus, and electronic equipment also using such a lighting apparatus.
Still further object of the invention is to provide a lighting apparatus which improves the utilization efficiency of the light by converting the light which has been reflected by the polarizer so that the light can pass through the polarizer and be transmitted to the liquid crystal display panel, a liquid crystal display using such a lighting apparatus, and electronic equipment also using such a lighting apparatus.